1. Field of the Invention
The present invention relates to a running method and a washing method for a spiral wound membrane element and a spiral wound membrane module employed for a membrane separator such as a reverse osmosis membrane separator, an ultrafiltration membrane separator or a microfiltration membrane separator.
2. Description of the Prior Art
Application of membrane separation is recently spread to water purification and waste water treatment, so that membrane separation is applied to hardly treatable liquid quality. In particular, recovery and recycling of industrial waste water through membrane separation is strongly demanded.
A hollow fiber membrane element is generally employed for such membrane separation in consideration of the membrane area (volumetric efficiency) per unit volume. However, the membrane of the hollow fiber membrane element is readily broken, and raw water is disadvantageously mixed into permeate to reduce separation performance when the membrane is broken.
Therefore, application of a spiral wound membrane element in place of the hollow fiber membrane element is proposed. The spiral wound membrane element can advantageously provide a large membrane area per unit volume similarly to the hollow fiber membrane element and maintain separation performance, and has high reliability.
When waste water containing a large amount of suspended, colloidal or dissolved matter is subjected to membrane separation, the suspended, colloidal or dissolved matter is deposited on a membrane surface as contaminants to reduce the water permeation velocity. Particularly in dead end filtration, contaminants are so readily deposited on the membrane surface that the water permeation velocity is remarkably reduced and it is difficult to continue stable filtration running.
In order to prevent the membrane surface from deposition of contaminants, cross flow filtration is performed. In this cross flow filtration, raw water is fed in parallel with the membrane surface for preventing the membrane surface from deposition of contaminants through shearing force caused on the interface between the membrane surface and fluid. In such cross flow filtration, a sufficient membrane surface linear velocity must be obtained for preventing the membrane surface from deposition of contaminants, and hence a sufficient flow rate of raw water must be fed in parallel with the membrane surface. When the flow rate of the raw water fed in parallel with the membrane surface is increased, however, the recovery per spiral wound membrane element is reduced and a large pump is required for feeding the raw water, while the system cost is remarkably increased.
On the other hand, contaminants deposited on the membrane surface may be removed by back wash reverse filtration. This back wash reverse filtration is generally performed in a hollow fiber membrane element.
For example, Japanese Patent Publication No. 6-98276 (1994) proposes application of back wash reverse filtration to a spiral wound membrane element. However, back pressure strength of a separation membrane of a conventional spiral wound membrane element is so low that the separation membrane may be broken when subjected to a back pressure in back wash reverse filtration. Therefore, the aforementioned gazette states that the spiral wound membrane element is preferably subjected to back wash reverse filtration with a low back pressure of 0.1 to 0.5 kg/cm2 (0.01 to 0.05 MPa).
According to an experiment made by the inventor, however, it was difficult to sufficiently remove contaminants when a spiral wound membrane element was subjected to back wash reverse filtration with such a back pressure, and it was impossible to maintain a high permeate flux over a long period.
The inventor has proposed a structure of and a method of preparing a separation membrane having back pressure strength of at least 2 kgf/cm2 in Japanese Patent Laying-Open No. 10-225626 (1998). However, it has not yet been sufficiently verified in relation to a spiral wound membrane element prepared with the separation membrane having such back pressure strength as to the level of a back pressure enabling back wash reverse filtration in practice and the range of the back pressure for back wash reverse filtration enabling the spiral wound membrane element to maintain a high permeate flux over a long period. Further, no verification has been made on a method of running a spiral wound membrane element including the aforementioned separation membrane having high back pressure strength and a method of running a spiral wound membrane module comprising such a spiral wound membrane element.
Also when the separation membrane having high back pressure strength is employed, stable filtration running cannot be continuously performed in a spiral wound membrane element and a spiral wound membrane module without reducing the permeate flux over a long period unless optimum washing conditions and an optimum washing method are applied and the filtration running is performed by an optimum method.